In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they are difficult to use as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and cycloolefin resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
Among others, a focus is drawn on (meth)acrylic resin base resists featuring a high resolution. One of the (meth)acrylic resins proposed thus far is a combination of (meth)acrylic units having methyladamantane ester as acid labile group units with (meth)acrylic units having lactone ring ester as adhesive group units as disclosed in JP-A. 9-90637. Acid labile groups of exo form are described in U.S. Pat. No. 6,448,420 (JP-A 2000-327633). These groups have so high an acid elimination ability and require a low level of activation energy for acid elimination, affording a high resolution and low dependence on post-exposure bake (PEB). Norbornane lactone is also proposed as an adhesive group having enhanced etching resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758. These studies have achieved significant improvements in the resolution of ArF resists.
The outstanding problem associated with the ArF lithography for forming a fine feature pattern is to meet both a high resolution and a minimized size difference between isolated and grouped patterns (known as I/G bias or proximity bias). So far as either one of high resolution and improved I/G bias is concerned, prior art techniques can achieve the goal to some extent. However, it is very difficult to meet both the requirements at a high level.
In general, resins for ArF resists have a structure in which carboxyl groups serving as the alkali soluble group are protected with acid labile groups, and perform such that the acid labile group is deprotected under the action of an acid to regenerate carboxyl groups whereby the resins become soluble in alkaline developers. It is noted that deprotection under the action of an acid is referred to as acid decomposition or “acidolysis.” A choice of an acid labile group having higher reactivity may be advantageous with respect to resolution, but enhance acid diffusion during acidolysis reaction, tending to degrade I/G bias. It is thus difficult to meet both resolution and I/G bias at a high level.